& M.  CctKVIin 


TherBlacn  Forms  Of  Sifter-  and  Me  rear 


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THE  BLACK  FORMS  OF  SILVER 
AND  MERCURY 

BY 

GEORGE  M.  CARVLIN 


THESIS 

FOR  THE 

DEGREE  OF  BACHELOR  OF  SCIENCE 

IN 

CHEMISTRY 


COLLEGE  OF  LIBERAL  ARTS  AND  SCIENCES 
UNIVERSITY  OF  ILLINOIS 


1921 


Digitized  by  the  Internet  Archive 
in  2015 


https://archive.org/details/blackformsofsilvOOcarv 


J. 


\ 5 1\ 

0.2$ 

UNIVERSITY  OF  ILLINOIS 

_ May__30,  _ r92  1 • 

THIS  IS  TO  CERTIFY  THAT  THE  THESIS  PREPARED  UNDER  MY  SUPERVISION  BY 

George  M.  Carvlin 

ENTITLED 

The  Black  Forms  of  Silver  and  Mercury 

IS  APPROVED  BY  ME  AS  FULFILLING  THIS  PART  OF  THE  REQUIREMENTS  FOR  THE 

DEGREE  OF  _ 

Bachelor  of  Science  in  Chemistry. 

(LN^Ejuu^. 

V/  Instructor  in  Charge 

Approved 

i£  A \ • 

HEAD  OF  DEPARTMENT  OF  Chemi_stry  . 

47G522 


ACKNOWLEDGMENT 


The  writer  wishes  to  express  his  sincere  thanks  and 
appreciation  to  Dr.  J.H.  heedy  for  his  advice  and  aid,  which  he  has 
rendered  in  a most  willing  and  courteous  manner. 


' 


, . 


5 . 


TABLE  OF  CONTENTS 


Page 

Acknowledgment.  1 

A.  SILVER 

I.  Introduction.  2 

II.  Historical.  2 

III.  Experimental.  3 

(a)  Methods  of  Preparation  of  Black  Silver  4 

(b)  Properties  of  Black  Silver  6 

(c)  Energy  Stresses  on  Black  Silver  8 

(d)  Analysis  of  Black  Silver  9 

B.  MERCURY 

I.  Historical.  11 

II.  Experimental.  11 

(a)  Methods  of  Preparation  of  Black  Mercury  11 

(b)  Properties  of  Black  Mercury  12 

IV.  Conclusion.  12 


Bibliography. 


13 


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A.  BLACK  SILVER 

I.  Introduction. 

Silver  is  capable  of  existing  in  a black  (amorphous)  form  possessing  qual- 
ities differing  greatly  from  those  of  normal  silver.  This  form  of  silver,  when 
pure,  is  a finely  divided  black  powder  somewhat  resembling  platinum  black  and  is 
broadly  distinguished  from  the  normal  form  by  color,  by  physical  properties  and 
by  certain  chemical  reactions. 

Black  silver  has  not  been  investigated  very  thoroughly  hitherto,  and  the 
preparation  of  this  substance  and  the  determination  of  its  properties  is  the  pur- 
pose of  this  investigation. 

II.  Historical. 

probably  the  earliest  record  of  experimental  work  on  black  silver  is 

found  in  a paper  published  by  Wohler  in  1629.  Wohler  describes  in  this  paper 

a black  compound  formed  by  the  reduction  of  AgNO^  by  various  organic  reagents, 

which  he  calls  silver  hemioxide  and  gives  the  formula  Ag  0.  He  was  substantiated 

4 

in  this  later  by  Von  Bihra. 

In  1882,  pillitz  published  two  papers.  He  commenced  by  disputing  the  pro- 
bability of  the  existence  of  Ag^O  on  the  grounds  of  valency;  namely,  as  implying 
that  oxygen  may  be  quadrivalent.  Pillitz  carefully  examined  the  so-called  hemi- 
oxide precipitated  by  alkaline  solutions  of  tin  and  could  find  no  trace  of  Ag^O 
in  it. 

The  first  persons  to  deny  logically  the  existence  of  Wohler’s  series  of 
hemi-compounds  of  silver  appear  to  have  been  Hewbury  and  lluthmann  working  inde- 
pendently some  time  later.  Both,  after  carefully  examining  Wohler’s  methods,  de- 


3 


Glared  the  red  solution  taken  by  the  latter  to  be  argentous  citrate,  to  be  a sus- 
pension  of  colloidal  silver.  At  the  present  time  the  formation  of  Ag40  by  V/8h- 
ler's  method  or  by  any  other  known  method  is  admitted  by  no  one;  so  his  results 
must  be  rejected. 

The  next  and  last  experimental  work  performed  on  this  subject  and  probably 
the  most  important  bearing  on  this  investigation,  was  that  of  M.  Carey  Lea  (1886- 
91).  In  a paper  published  in  the  American  Journal  of  Science  for  1889  he  described 
three  distinct  allotropic  forms  of  silver.  Lea,  however,  does  not  furnish  positive 
proof  of  the  existence  of  these  three  separate  forms  and  I shall  later  attempt  to 
point  out  some  defects  in  his  methods. 

III.  Experimental. 

Lea,  in  his  work  upon  this  subject  inferred  that  silver  may  exist  in  three 
distinct  forms;  1st,  allotropic  silver  which  may  be  yellow,  blue  or  green  or  may 
have  almost  any  color  and  may  be  soluble  or  insoluble  in  v/ater;  2nd,  the  intermed- 
iate form  which  may  be  yellow,  red  or  green  and  is  almost  as  indifferent  chemically 
as  ordinary  silver;  3rd,  ordinary  silver. 

After  preparing,  according  to  the  method  described  by  Lea,  the  allotropic 
form  of  silver  soluble  in  water,  there  remained  in  the  solution  a colloidal  form 
in  the  suspensoid  state.  A precipitation  of  this  form  of  silver  (which  we  nor/  re- 
cognize as  '’colloidal")  takes  place  both  under  the  action  of  light  and  at  increased 
temperatures.  This  precipitate  is  black  and  is  not  in  the  least  soluble  in  water. 

in  order  to  study  the  properties  of  the  second  modification  of  silver  de- 
scribed by  Lea,  I precipitated  silver  nitrate  with  a solution  of  ferrous  sulphate 
and  sodium  citrate,  decanted  the  liquid  and  then  "dissolved  " the  precipitate  in 
water,  the  peptization  being  possibly  due  to  the  ferric  hydroxide  formed  from  the 
ferrous  sulphate.  On  the  addition  of  ammonium  nitrate  the  silver  separated  out. 


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I filtered  off  this  second  precipitate,  washed  it  with  water  containing  some  am- 
monium nitrate,  and  finally  with  alcohol  before  drying  it  over  sulfuric  acid.  The 
silver  so  prepared,  though  not  entirely  pure,  due  probably  to  contamination  v/ith  a 
small  amount  of  citric  acid,  is  black  and  is  no  longer  soluble  in  water. 

The  preparation  of  black  silver,  which  is  really  coagulated  colloidal  sil- 
ver, consists  in  reducing  solutions  of  silver  salts  v/ith  suitable  reducing  agents. 
The  methods  which  I have  used  are  as  follows; 


a.  Methods  of  preparation. 

1.  Electrolytic  Method. 


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trode.  By  using  a normal  solution  of  KCl  as  electrolyte 
and  a silver  anode  and  jplatinum  cathode,  I obtained  an  even 
deposit  of  AgCl  on  the  anode.  The  average  E.M.F.  was  8 volts 

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and  the  average  current  0.5  amperes.  TThen  the  deposit  be- 
came fairly  heavy,  I changed  the  electrolyte  to  N/l  KOH  and  reversed  the  current, 
making  the  silver  chloride  electrode  the  cathode  and  the  platinum  the  anode.  In 
this  manner,  the  AgCl  was  reduced  to  black  silver,  forming  a smooth  black  metallic 
surface  on  the  electrode. 

I tried  to  prepare  black  silver  by  this  method  using  freshly  precipitated 
AgCl  for  the  cathode  instead  of  the  silver  chloride  plate.  This  method  was  un- 
successful however,  due  to  the  low  conductivity  of  AgCl;  that  is,  the  solid  silver 
chloride  did  not  function  as  part  of  the  cathode. 

2.  Reduction  of  Silver  Nitrate  with  Ferrous  Citrate. 

This  reaction  is  slow  and  gives  very  low  yields.  In  alkaline  solution, 


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black  silver  will  be  formed  and  in  neutral  or  acid  solution,  normal  silver  will 
come  down.  The  method  was  discarded  as  impractical.  ^ 

3.  Reduction  with  Stannous  Nitrate. 

The  silver  prepared  by  this  method  first  forms  as  a red  colloidal  suspen- 
sion. Upon  standing  for  about  half  an  hour,  this  precipitate  coagulates  and  turns 
first  gray  and  then  black.  This  coagulation  may  be  hastened  by  adding  a slight 
amount  of  sodium  or  ammonium  hydroxide  or  tartaric  acid. 

The  product  prepared  by  this  method  is  very  pure  and  the  yields  obtained 
are  satisfactory. 

4.  Reduction  with  Ferrous  Oxalate. 

This  method  of  reduction  is  very  slow  and  not  at  all  satisfactory.  The 
same  difficulties  are  met  with  in  all  reductions  in  organic  acid  solution. 

5.  Reduction  of  Silver  Chloride  with  Stannous  Chloride. 

The  silver  formed  by  this  method  is  very  impure,  as  it  seems  to  be  adsorbec 
by  Sn(0H)4  formed  in  the  solution  in  the  same  manner  as  purple  of  Cassius  is 
formed  when  colloidal  gold  is  adsorbed  by  stannic  hydroxide.  If  the  SnCl^  is 
ground  up  finely  and  an  excess  of  NH^OE  is  added,  the  black  silver  formed,  when 
thoroughly  washed  with  NH^NO^  and  then  with  C2HgOH  shows  a purity  of  about  94.5%. 
Black  silver  may  never  be  obtained  in  a pure  state  by  this  method,  however,  be- 
cause it  is  not  possible  to  wash  out  a considerable  amount  of  the  stannic  hydrox- 
ide. It  seems  that  this  phenomenon  is  one  of  mutual  coagulation  of  colloids. 
Colloidal  silver  is  a negative  colloid  (that  is,  the  suspensoid  migrates  to  the 
anode)  and  stannic  hydroxide  is  a positive  colloid,  it  may  be  assumed  that  these 
colloids  combine  to  form  some  sort  of  a physico-chemical  complex  which  is  quite 


stable 


6 


6.  A solution  of  tannin  (or  dextrine)  in  10%  NaOH  was  added  to  AgNO  in 

3 

excess.  The  black  silver  this  formed  was  a fine,  amorphous  precipitate  and  the 
average  analysis  of  six  samples  was  97.26%.  Before  analysis  these  samples  were 
washed  repeatedly  with  NH^NO^  and  finally  with  C^EgOH  after  which  they  were  dried 
at  60°  C. 

From  the  above  data  it  was  concluded  that  the  best  non-elec trolytic  methods 
for  preparing  black  silver  are  3 and  6.  From  the  results  obtained  it  seems  reason- 
able to  conclude  that  when  silver  salts  are  reduced  in  alkaline  or  weak  organic 
acid  solution,  black  silver  is  formed;  while,  if  the  reduction  takes  place  in  acid 
solution,  normal  gray  silver  is  formed. 

According  to  the  thermodynamic  point  of  view,  it  requires  a higher  voltage 
to  transfer  a gram  of  silver  from  the  ionic  to  the  metallic  form  when  the  silver 
i^on  concentration  in  the  solution  is  low  than  when  it  is  high.  The  precipitation 
of  silver  from  alkaline  or  organic  acid  solution,  therefore,  involves  high  poten- 
tial effects,  or  what  corresponds  to  the  same  thing,  high  energy  effects  or  chemi- 
cal work;  consequently  silver  precipitated  from  solutions  of  low  silver  ion  con- 
centration may  have  a much  higher  energy  content  than  ordinary  silver.  That  is  to 
say,  the  silver  may  form  in  an  allotropic  form  which  is  more  or  less  unstable. 

b.  Properties  of  Black  Silver. 

1.  The  color  Reaction. 

When  amorphous  silver  is  immersed  in  a solution  of  a substance  readily 
parting  with  oxygen,  sulfur  or  halogen,  a varied  colored  film  is  formed.  The  col- 
oring of  other  metals  by  a film  of  sulfide  or  oxide  is  familiar.  T7ith  black  silver 
the  colors  are  extremely  brilliant. 


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The  substances  which  produce  these  reactions  are  potassium  ferricyanide  and 
permanganate,  ferric  chloride,  alkaline  hypochlorites  and  sulfides,  solution  of 
iodine,  etc.  A five  or  ten  percent  ferricyanide  solution  is  the  best  of  these  re 
agents  because  its  reaction  is  more  distinctive.  Moreover,  the  action  of  the  lat 
ter  on  normal  silver  is  very  slight  and  here  we  have  a means  of  detecting  the  two 
forms.  The  composition  of  the  colored  film  is  probably  silver  ferrocyanide , sil- 
ver ferricyanide,  or  a combination  of  the  two. 

2.  Formation  of  a Continuous  Film  Upon  Drying. 

If  black  silver  is  taken  in  a pasty  condition  and  is  spread  evenly  over 
glass  with  a fine  brush  and  dried  slowly,  a perfect  black  mirror  is  formed. 

3.  The  Halogen  Reaction. 

7, lien  this  amorphous  form  of  silver  is  brushed  over  paper  and  the  resulting 
film  is  exposed  to  the  action  of  any  haloid  in  solution,  very  beautiful  colora- 
tions are  obtained.  The  experiment  succeeds  best  with  substances  which  easily 
give  up  halogen,  such  as  sodium  hypochlorite,  ferric  chloride,  iodine  dissolved 
in  potassium  iodide,  etc. 

4.  The  Action  of  Acids. 

The  stronger  acids  easily  convert  the  black  form  of  silver  into  the  normal 
gray  silver;  even  acetic  acid,  not  too  much  diluted,  does  this. 

5.  Physical  Condition. 

This  black  amorphous  silver  is  easily  reduced  by  grinding  or  pressure  to  an 
impalpable  powder. 

6.  Amalgamation  with  Mercury. 

Black  silver,  especially  that  prepared  by  the  first  method  above,  easily 


8 


forms  amalgams  with  mercury,  having  a smooth,  uniform  surface  exactly  like  nor- 
mal silver  amalgams. 

c.  Action  of  energy  on  black  silver. 

1.  Action  of  Electricity. 

High  tension  electricity  instantly  converts  colloidal  silver  into  the  nor- 
mal gray  form.  A plate  of  black  silver  prepared  electrolytically  was  held  in 
the  path  of  a spark  of  about  one  inch  and  gray  spots  were  noticed  on  the  plate 
after  each  discharge.  The  same  effect  was  produced  upon  a film  of  black  silver 
on  paper.  That  the  gray  spots  formed  in  this  way  are  normal  silver  is  easily 
proved  by  immersing  the  piece  in  a dilute  solution  of  potassium  ferricyanide  or 
sodium  hypochlorite.  The  part  acted  upon  by  electricity  is  not  affected,  while 
the  rest  of  the  film  shows  the  coloration  characteristic  of  colloidal  silver. 

2.  Action  of  Heat. 

V/hen  films  of  black  silver  on  glass  or  plates  formed  by  the  first  method' 
above  are  kept  at  100°  C.  for  eight  or  ten  hours,  there  is  observed  a border  of 
normal  silver.  At  higher  temperatures,  the  change  is  much  more  rapid.  Here 
again  the  ferricyanide  test  showed  complete  conversion. 

3.  Action  of  Mechanical  Force. 

The  slightest  application  of  force  suffices  to  instantly  convert  amorphous 
silver  to  its  normal  form.  If  black  silver  is  spread  over  a clay  plate  to  dry 
and  is  rubbed  lightly  with  a hard  surface  or  even  with  the  finger,  the  black 
film  is  instantly  converted  to  bright  metallic  silver. 


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4.  Action  of  Light. 

Whan  films  of  colloidal  silver  are  exposed  to  direct  sunlight,  no  effects 
may  be  noticed  for  some  time.  After  about  a month  of  this  exposure,  the  sample 
used  had  turned  to  a dull  gray  color;  in  two  months  time  this  color  was  slightly 
more  intensified  but  the  sample  was  not,  as  we  had  expected,  completely  converted 
to  normal  silver.  It  had  lost  its  activity  toward  ferricyanide  solution,  however, 
and  from  this  I am  led  to  believe  that  a slow  conversion  was  taking  place  and 
that,  with  sufficient  exposure  the  sample  would  have  been  completely  converted. 

This  phenomena,  I believe,  explains  the  gradual  fading  of  silver  photograph- 
ic prints.  If  these  prints  are  allowed  to  stand  for  several  months  or  years,  the 
image  is  gradually  obscured  due  to  the  conversion  of  the  silver  from  its  black 
amorphous  form  to  the  ordinary  form.  This  difficulty  is  overcome  in  practice  by 
toning  the  prints  with  gold  or  platinum,  thus  replacing  the  silver  by  these  more 
permanent  elements. 

d.  Analysis. 

The  purest  samples  of  black  silver  which  I was  able  to  obtain  contained 
about  97 Jo  silver,  the  rest  being  occluded  tin  in  the  third  method,  and  organic 
acids  in  the  other  methods.  Black  silver  prepared  by  the  electrolytic  method 
seems  to  be  the  purest  obtainable,  however,  and  upon  this  was  based  the  most  con- 
clusive evidence  of  the  existence  of  pure  silver  in  the  black  form. 

I determined  the  electrode  potential  of  a uniform  plate  of  black  silver 
and  found  it  to  be  0.6976  volts,  while  that  of  normal  metallic  silver  was  found 
to  be  0.6988  volts.  The  divergence  of  0.001H  volts  is  within  the  experimental 
error,  so  that  black  silver  is,  without  a doubt,  pure  silver. 

This  result  was  rather  surprising,  as  I expected  that  the  more  reactive 


10 


amorphous  form  of  silver  would  have  a decidedly  lower  electrode  potential  than 
the  normal  form. 


11 


B.  BLACK  MERCURY 

I.  Historical. 

The  fact  that  mercurous  salts  give  black  derivatives  with  ammonia  is  one 

of  the  earliest  records  of  chemistry.  It  is  surprising,  therefore,  that  so  little 

attention  has  been  given  to  the  ultimate  nature  of  the  black  substance.  There  are 

in  general  two  theories  to  account  for  this  phenomenon:  First,  the  formation  of 

a black  dimercuro-derivative , as  “ 17H2;  second,  the  breaking  down  of  the  mer- 

Hg  - Cl 

curous  nucleus  ^ “ into  Hg  = and  free  mercury,  the  latter  being  black.  This  view 
Hg  - 

is  borne  out  by  such  phenomena  as  the  following.  If  a smear  of  the  black  precipi- 
tate formed  by  the  action  of  ammonium  hydroxide  on  calomel  is  placed  on  a glass 
slide,  the  color  gradually  changes  over  to  a gray,  with  a steady  loss  in  weight. 
This  is  attributed  to  the  loss  of  the  black  mercury  by  volatilization. 

II.  Experimental . 

a.  Methods  of  preparation. 

1.  Stannous  nitrate  reduction. 

2.  Formation  of  a complex. 

In  the  latter  method,  the  mercury  complex  comes  down  as  a fine  white  pre- 
cipitate and  partially  obscures  the  black  mercury  formed. 

3.  Black  mercury  may  also  be  prepared  by  adding  a solution  of  dextrine  or 
tannin  in  NaOE  to  a solution  of  Eg^dTCLL. 

Ct  O & 


The  first  and  third  methods  seem  to  give  the  largest  yields  and  the  purest 


12 


product.  The  average  analysis  of  six  samples  formed  by  the  last  method  gave  96. 87^ 
mercury. 

I am  inclined  to  believe  that  the  black  product  obtained  by  exposing  mer- 
curous chloride  to  sunlight  is  black  colloidal  mercury,  though  it  may  be,  as  Lea 
suggests,  a mercury  subchloride. 

b.  properties  of  black  mercury. 

The  properties  and  reactions  of  black  mercury  are  the  same  as  those  given 
for  black  silver.  One  interesting  property  of  colloidal  mercury  is  that  of  amal- 
gamating with  gold  and  silver.  It  forms  these  amalgams  as  readily  as  does  normal 
mercury,  and  they  are  uniform  and  smooth  and  exactly  the  same  in  every  way  as  nor- 
mal amalgams. 

IV.  Conclusion. 

From  the  results  obtained  in  this  investigation,  I have  concluded  that  the 
vari-colored  substances  obtained  by  Lea  were  not,  as  he  inferred,  allotropic  forms 
of  silver  but  rather  colloidal  silver  in  different  stages  of  coagulation.  The 
fact  that  both  forms  of  silver  have  identical  electrode  potentials  indicates  that 
the  black  silver  may  not  be  in  the  allotropic  condition,  but  rather  is  in  a com- 
pletely coagulated  colloidal  state. 

Whether  silver  shall  be  reduced  from  its  compounds  in  the  metallic  or  amor- 
phous form  depends  upon  the  reducing  agent  applied;  so  that  it  cannot  be  said  with 
any  certainty  in  which  condition  it  is  present  in  its  compounds. 

Lea,  in  concluding  his  work  on  this  subject,  suggests  that  in  the  black 
form,  silver  is  in  the  atomic  condition  and  that,  in  the  conversion  from  the  black 
to  the  normal  form  which  is  in  the  molecular  condition,  a polymer izat ion  takes 


place . 


13 


It  seems  more  probable,  however,  in  view  of  the  greater  reactivity  of  the 
black  form,  that  the  latter  is  merely  a different  condition  of  the  same  silver  and 
that  in  the  conversion  to  normal  silver,  no  essential  change  in  the  structure  of 
the  molecule  takes  place. 

Much  useful  work  may  be  accomplished  in  finding  practical  applications  for 
these  two  colloidal  substances,  probably  as  a catalyst  to  replace  the  more  expen- 
sive forms  of  platinum  and  palladium.  I tried  black  silver  and  mercury  as  a cata- 
lyst in  connection  with  various  organic  reductions  in  which  platinum  black  has 
been  used  heretofore,  but  did  not  achieve  any  noticeable  success.  Both  will  de- 
compose hydrogen  peroxide,  but  much  more  slowly  than  platinum  black. 

These  two  elements  are  not,  in  all  probability,  the  only  ones  possessing 
these  peculiar  amorphous  forms.  Copper,  bismuth  and  some  other  metals  give  pro- 
mise of  existing  in  more  than  one  form. 


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B IBLIOGRAPHY 

1.  American  Journal  of  Science,  37,  476  (1889). 

2.  American  Journal  of  Science,  41,  179,  259  (1891). 

3.  J.  Prokt  Ghem. , 38,  588  (1888). 

4.  2.  Anorg.  Chem. , _21,  361  (1889);  23,  131  (1900). 


r UNIVERSITY  OF  ILLINOIS-URBANA 


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